1. Field of the Invention
The invention relates generally to load elevators for use in loading and unloading objects; in particular, it relates to load elevators that can self-adjust to maintain the level of a changing load at a convenient predetermined height.
2. Description of the Prior Art
In the process of handling objects, such as packages in a warehouse, the objects are commonly transferred manually from a pallet resting on the floor to a table, conveyor, etc., or vice versa. While the table or conveyor remains at a fixed height, the top of the load on the pallet varies in height as the objects accumulate on or are removed from the pallet. This varying elevation of the objects to be handled is fatiguing and can be hazardous for the person doing the moving. Therefore, elevators have been developed for raising the pallet from the floor to a more convenient height and also for automatically adjusting the height of the pallet to a varying optimal position as the load increases or decreases.
For example, U.S. Pat. No. 4,764,075 describes a scissor elevator supported by helical metal springs that maintain the top of the at a preset height above the floor as boxes are added or removed. U.S. Pat. No. 5,299,906 describes a self-adjusting pneumatic scissor elevator with an air actuator system that includes a compressible air actuator or bellows and a fixed-volume reservoir. The bellows, mounted between the scissors linkage and the load platform, is compressible between specified maximum and minimum bellows heights that correspondingly determine substantially different maximum and minimum bellows volumes. The air reservoir is coupled to the bellows and has a fixed volume that is substantially larger than the difference between the maximum and minimum volumes of the bellows. As a result, during loading or unloading the height of the platform changes so as to maintain the top of the load at substantially the same level while objects are being added to or removed from the platform.
As evidenced by its commercial success, the load elevator of the '906 patent represented a significant improvement in the art. However, once the pressure in the pneumatic system is set, the performance of the elevator is fixed according to a predetermined height-versus-load curve that depends, in large part, on the volume of the reservoir. In the field, the operator is normally not allowed to change the system pressure or, if permitted to do so, a pressurized source of air may not be readily available. Therefore, because the performance curve of the elevator is fixed for a given system pressure regardless of the density of the load being handled, the height of the platform cannot be optimal for all weights. That is, heavier objects (those with a higher density) will lower the platform more rapidly than lighter objects. If the elevator pressure is set for a lighter load, this means that the operator will have to work at a lower height than would be optimal if the pressure were set for the heavier load (and vice versa).
This is a problem in environments where the loads being handled vary materially from shipment to shipment. For example, referring to FIG. 1, an elevator like the one described in the '906 patent is designed to produce a travel of 20 inches from the top elevation of the platform (at a height of 30 inches) to the bottom elevation (at a height of 10 inches). Obviously, the maximum load required to produce the complete lowering of the platform depends on the pressure of the pneumatic system. As indicated by the first curve on the left of the figure, the platform will reach it lowest height when the load is 500 pounds if the system pressure is set at 16 psi. As the load on the platform increases, the travel of the platform is roughly linear, which is the desired performance if the density of a uniform load is such that the platform is fully loaded when 500 pounds of material are placed on it. If, on the other hand, the material is twice as dense, for instance, it is clear that the platform will reach its lowest point (at 10 inches of height) when the platform is only half loaded, which means that most of the time the operator will be working at a lower height than optimal.
The reverse problem occurs if the pressure is set high for heavier loads and a lighter one is handled instead. Referring to the last curve on the right in FIG. 1, for example, with a system pressure set at 75 psi the platform would begin descending at 800 pounds and would reach it lowest point only when the load is at 2,150 pounds. Thus, if the weight of the material being handled were such that 500 pounds of it would be sufficient to completely load the platform, the operator would have to place the material at an increasingly higher elevation over a platform that would remain at the constant elevation of 30 inches (because it would not start moving until 800 pounds were placed on it).
This can be a serious drawback when the operator cannot change the system pressure to conform to the requirements of the job at hand. Therefore, it would be very useful to be able to operate a system charged with a given initial pressure so as to change the load required to lower the platform to its lowest height. That is, it would be very advantageous to be able to combine the performance curves of FIG. 1 to obtain multiple maximum-load capabilities from a common minimum-load initial pressure. The present invention is directed at solving this problem by providing a self-contained system with optional settings that allow the operator to handle loads of materially different density at substantially the same work elevation throughout the range of motion of the platform.